Early solid golf balls were generally two piece balls, i.e., comprising a core and a cover. More recently developed solid balls are comprised of a core, a core layer or mantle layer, and a cover, in order to improve the playing characteristics of the ball.
The prior art is comprised of a variety of golf balls that have been designed to provide particular playing characteristics. These characteristics are generally the initial velocity and spin of the golf ball, which can be optimized for various types of players. For instance, certain players prefer a ball that has a high spin rate in order to control and stop the golf ball. Other players prefer a ball that has a low spin rate and high resiliency to maximize distance. Generally, a golf ball having a hard core and a soft cover will have a high spin rate. Conversely, a golf ball having a hard cover and a soft core will have a low spin rate. Golf balls having a hard core and a hard cover generally have very high resiliency for distance, but are hard feeling and difficult to control around the greens. Various prior art references have been directed to adding a mantle layer or second cover layer to improve the playability of solid golf balls.
The spin rate of golf balls is the end result of many variables, one of which is the distribution of the density or specific gravity within the ball. Spin rate is an important characteristic of golf balls for both skilled and recreational golfers. High spin rate allows the more skilled players, such as PGA professionals and low handicapped players, to maximize control of the golf ball. A high spin rate golf ball is advantageous for an approach shot to the green. The ability to produce and control backspin to stop the ball on the green and side spin to draw or fade the ball substantially improves the player's control over the ball. Hence, the more skilled players generally prefer a golf ball that exhibits high spin rate.
On the other hand, recreational players who cannot intentionally control the spin of the ball generally do not prefer a high spin rate golf ball. For these players, slicing and hooking are the more immediate obstacles. When a club head strikes a ball, an unintentional side spin is often imparted to the ball, which sends the ball off its intended course. The side spin reduces the player's control over the ball, as well as the distance the ball will travel. A golf ball that spins less tends not to drift off-line erratically if the shot is not hit squarely off the club face. The low spin ball will not cure the hook or the slice, but the lower spin will reduce the adverse effects of the side spin. Hence, recreational players prefer a golf ball that exhibits low spin rate.
There is a significant need in the industry for a ball having a large diameter but low compression core and surrounded by a harder outer core layer, i.e. a relatively large thermoplastic core with a thermoset core layer about it. As used herein, the term “thermoset” material refers to an irreversible, solid polymer that is the product of the reaction of two or more prepolymer precursor materials. However, traditionally there have been significant manufacturing difficulties when trying to compression mold a thermosetting polybutadiene-comprising layer over a thermoplastic core layer. Typical results include the melting, distortion and flow of the thermoplastic core into the surrounding rubber layer. These problems can be due to the temperature, time and pressure required to thoroughly cure the rubber. The temperatures required usually range between 315 and 350° F. which can be significantly higher than the melting temperature of the underlying thermoplastic layer. Additionally, the exotherm generated from the curing reaction of a 11-15 minute cure can elevate the rubber temperature even higher. Thus, there remains a need to minimize the temperature and/or the time that the thermoplastic material is exposed. Ball construction materials, which teach the use of a heat resistant layer, are needed to allow the compression molding of a thermoset layer over a thermoplastic layer without significant flow of the thermoplastic layer into the thermoset layer.
Other prior art golf balls have multiple core layers to provide desired playing characteristics. For example, U.S. Pat. No. 6,815,480 discloses blends of highly neutralized polymers (HNP) with hytrel and other high melt temperature materials but not discrete layers of high melt materials.
U.S. Pat. Nos. 6,057,403, 6,213,895, and 6,585,608, issued to Sullivan, disclose thermoplastic core layers but do not disclose highly neutralized polymers or the use of a heat resistant outer core layer such as disclosed in this invention.
U.S. Pat. No. 6,450,901 discloses outer covers comprising blends of very low modulus ionomers (VLMI) with a high melt ionomer (Surlyn 8549, vicat of 84° C.) having a vicat softening temp of 74° C. or more, and preferably 84° C. or more. However, no disclosure of a discrete layer of the high vicat material or of HNPs is made.
These and other objects and features of the invention will be apparent from the following summary and description of the invention, the drawings and from the claims.